Precision timing distribution is important for future accelerator facilities and, in particular, for precise synchronization between low-level radiofrequency (RF) systems in such facilities as well as to the extraction of microwave signals from optical clocks.
Seeding of free-electron lasers operating in the extreme ultraviolet and soft X-ray regime with radiation generated via high harmonics from noble gases may result in a fully coherent X-ray laser. For seeding of such large-scale facilities spanning over several hundreds meters, it is critical to synchronize lasers and radiofrequency systems with low (preferably sub-femtosecond range) timing jitter in a long-term stable arrangement.
To achieve this synchronization, the pulse repetition rate of an optical master oscillator implemented as a mode-locked laser is stabilized to a frequency standard or to an ultra-low noise microwave oscillator that is clocking the facility. The pulse train is distributed to all critical sub-systems by use of timing-stabilized fiber links; i.e., the pulse trains leaving different fiber links are synchronous. The radiofrequency or optical sub-systems are then synchronized to the pulse trains at the fiber outputs.
It has been shown that the extraction of a microwave signal from an optical pulse train emitted by a mode-locked laser using direct photo-detection is limited in precision by excess phase noise [see E. N. Ivanov, et al., “Analysis of Noise Mechanisms Limiting the Frequency Stability of Microwave Signals Generated with a Femtosecond Laser,” IEEE J. Sel. Top. Quant. Elec. 9, 1059-1065 (2003), and A. Bartels, et al., “Femtosecond-Laser-Based Synthesis of Ultrastable Microwave Signals from Optical Frequency References,” Optics Letters 30, 667-669 (2005)].
The origin of this excess noise has been identified as amplitude-to-phase conversion in the photo-detection process, beam-pointing variations, and pulse distortions by photodetector nonlinearities. In addition to this excess phase noise and timing jitter by photodetector nonlinearities, the long-term synchronization stability is limited by the temperature dependence of semiconductor photodiodes. Thus, a new synchronization scheme to avoid these problems is highly desirable.
The great potential of mode-locked lasers for generating ultra-low-jitter radiofrequency signals has been recognized. Recently, the extraction of a radiofrequency signal from an optical pulse train emitted by a mode-locked laser using direct photo-detection has been shown to be limited in precision by excess phase noise. The major origin of this excess noise has been identified to be the amplitude-to-phase (AM-to-PM) conversion in the photodetector. The inventors have measured the AM-to-PM conversion factor at ranges from 1 to 10 picoseconds per milliwatt (ps/mW), depending on the bias voltage and diode types. The intensity noise of the laser can be converted into a significant amount of phase noise by this conversion process. The inventors previously demonstrated a scheme to avoid this conversion by transfer of timing information in the optical domain based on a free-space Sagnac interferometer, as described in US Patent Application Publication No. 2005/0265406 A1.